Naming the chopping modes and ON/OFF  #148

To match other radio-astronomy software (e.g., CLASS for APEX), we should consider re-defining what is meant by ON/OFF.

Current status:
AB chop: State = ON, source: beam = B, sky: beam = A
BA chop: State = OFF, source: beam = A, sky: beam = B

Proposed modification:

State = AB: AB chop, source: beam = ON. sky: beam = OFF
State = BA: BA chop, source: beam = ON. sky: beam = OFF

This way, the source and off-source data streams are consistently labeled.